Pixel Circuit Threshold Voltage Compensation for OLED Uniformity
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Solution Overview
Problem
Organic light emitting display devices face issues with non-uniform brightness due to variations in threshold voltages of driving transistors, which affect image quality and power consumption.
Innovation Solution
A pixel circuit is designed with a specific configuration of transistors and capacitors that compensates for the threshold voltage of the driving transistor, including an organic light emitting diode coupled between two power supplies, and a set of transistors and capacitors that control the driving current and node voltages to maintain uniform brightness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a simple pixel circuit is used, then device complexity is reduced, but threshold voltage variations cause non-uniform brightness
Solution Approach 1:
The pixel circuit performs preliminary action by storing the threshold voltage compensation value in the first capacitor before the actual image display. The compensation value is calculated during the charge storage period when the fourth transistor is on, and then held in the first capacitor during the emission period, allowing the driving transistor to operate with compensated threshold voltage without requiring complex real-time adjustment circuits
Solution Approach 2:
The pixel circuit implements feedback by using the fifth transistor to sense the actual threshold voltage of the driving transistor and feeding this information back to the first capacitor for storage. The second capacitor stores the difference between the reference voltage and the actual threshold voltage, creating a feedback loop that continuously compensates for threshold voltage variations to maintain uniform brightness
2Manufacturing precision
If threshold voltage compensation is implemented, then brightness uniformity is improved, but power consumption increases
Solution Approach 1:
The pixel circuit employs periodic action by operating the fourth transistor only during the charge storage period to transfer the compensation value to the first capacitor, while keeping it off during the emission period. This periodic operation of the compensation circuitry allows brightness uniformity to be maintained without continuously consuming power, as the compensation values are stored in capacitors that maintain their charge without additional power input
3Reliability
If more transistors are added for compensation, then threshold voltage control is improved, but device complexity increases
Solution Approach 1:
The pixel circuit achieves multi-functionality by having the first capacitor serve dual purposes: storing the compensation value during the charge storage period and maintaining it during the emission period. The second capacitor similarly stores the voltage difference and maintains it throughout the cycle. This allows the compensation mechanism to perform both calculation and storage functions without requiring separate dedicated circuits for each function, thereby reducing the overall number of transistors needed
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution improves image quality by ensuring uniform brightness and reduces power consumption by effectively managing the driving current and threshold voltage variations, preventing blurring and optimizing power usage.
Implementation Method 1
organic light emitting diodes that generate light by the recombination of electrons and holes
Data Source
AI summary
A pixel includes: an organic light emitting diode coupled between a first power supply and a second power supply; a first transistor coupled between the organic light emitting diode and the second power supply; a second transistor coupled to a first node to which a gate electrode of the first transistor is coupled; a first capacitor coupled between the first node and a second node; a third transistor coupled between the second node and a data line; a fourth transistor coupled between the first node and the second node; a fifth transistor coupled between the first transistor and the second power supply; and a second capacitor coupled between the second node and a third node between the first transistor and the fifth transistor.